“Battery capacity” is a measure (typically in ampere-hours) of the charge stored by the battery, and is determined by the mass of active material contained in the battery. The battery capacity represents the maximum amount of energy that can be extracted from the battery under certain specified conditions.
The effective storage capacity of the battery, however, diminishes with age and undergoes irreversible damage. This damage is caused by corrosion and other irreversible chemical processes. Aging of the internal battery components contributes to the damage as well. The plates of the cells corrode, and as the plates corrode, their operational surface area diminishes and the electrolyte undergoes chemical changes, causing them both to be less chemically reactive. The changes reduce the volume of reactive components in the cell, reducing the charge capacity of the cell. It also increases the internal resistance of the cell, as the corrosion products inhibit the free flow of electrons through the plates. Each charge/discharge cycle of the battery also has a similar effect but at an accelerated rate. As a result, as the battery ages and deteriorates, the effective capacity of the battery decreases, reducing the amount of time the battery can supply power to a device. In addition, the voltage level drops off faster, applying a decreasing voltage to the device.
One indicator of the battery's ability to retain charge is the battery “state of health.” Many applications use this parameter to estimate battery performance, for example the “run-time” of the battery, which reflects the amount of time the battery will continue to provide power before it dies. An accurate estimate of the run-time is desired to provide alerts to a user.
Conventional methods for measuring the effective capacity of the battery are inefficient and prone to errors because many methods require removal of the battery from the device to measure various parameters that are affected by aging.